I am working on an web app, part of which grab contents from other several sites and show them in a merged way (partial info). Those sites are already dynamic and thus it will be super dynamic/changeable in every minutes. I was wondering to know, whether I can have seo benefits in this way, (as these are already exists in those external sites)?

If doing some anything can help to get SEO on these pages, can you please explain how to achieve a decent SEO effect for these pages?

Just to clarify one mis-understanding, I am not stealing the contents from other sites.I am using those site's api access to do this and its very much allowed. My main intention is to merge only the CLOSELY RELATED items into one place from several sites(using their api, of course), which will facilitate visitors to see them all in one place and save time from explore each sites separately. Instead using full contents, I am showing partial, so that, if a user get interested, then they can explore the full content page from their real owner sites and I get the credit for affiliation. Hope this make sense...

Ideals like these won't make you money or will you rank high for these. Sites like these are likely to get emails telling you to take of their text of your site. Text is copyrighted just as well as Movies, Mp3s and so on! even if you back link to the source you still don't have permission, while some may not mind others do.. IMDB before example! Once upon a time these sites worked but Google clamped down and now your just be nowhere to see in the rankings!
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Simon Hayter♦Feb 6 '13 at 0:38

Thanks, I have clarified more. Hope this will help you understand better about my intention.
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RanaFeb 6 '13 at 6:19

This way, should you run into problems, it will hopefully affect only one part (which is how some sites, hit by recent algorithms, try to re-structure or survice).

Hints like rel=canonical won't help you much, as source is just a fragment of the content on domain A und target (the canonical content) is on a different domain with more than slightly differences in content. Also make use of webmastertools of Google and Bing and watch out for notices that should concern you.

So if the intention of your site is clear and the service offers great benefit, a good search engine will honor that and you'll see benefit from that.

Grabbing content from other sites is called "scraping". Scraping is not a good strategy for SEO rankings. Google has stated that they don't want sites in their index that only aggregate content from other places or show nothing but links and results from other sites. To be eligible to be considered for the Google index you need original content or your own commentary and point of view.

The most likely way that you will be discovered is through duplicate content. Google has algorithms that can detect that text is the same on two websites. They then try to show one of those sites and penalize the other.